Graphite, which has long been known as a carbon material having many excellent properties, such as high electrical conductivity, heat resistance, lightness, low thermal expansivity, high thermal conductivity, and self-lubricating properties, has been used in many applications. In addition, new carbonaceous materials similar to graphite, such as graphite sheets obtained by carbonizing resin materials by burning and nano-carbon materials including fullerene and carbon nanotubes, have recently been discovered to have many excellent properties that graphite also has and increasingly used in wide variety of applications. More recently, methods for producing graphene, which constitutes part of graphite and has a thickness on the order of nanometers and a structure in which numbers of benzene ring structures are two-dimensionally arranged in an adjacent manner, and thin layer graphite, which is formed of multi-layer graphene, have been discovered, and research and development has actively been conducted to use these new materials.
Graphene and thin layer graphite have many excellent physical properties, such as high strength, high elasticity, high mobility, high gas barrier properties, and good light transmission due to being thin, as well as high electrical conductivity and high thermal conductivity, which graphite also has, and are also chemically stable. Thus, their applicability wider than that of graphite has been studied. Methods for producing graphene and thin layer graphite are roughly classified into three methods, specifically as follows:
i) A gas phase method, in which a very thin graphene of a single layer or a few layers is formed by chemical vapor deposition (CVD).
ii) A solid phase method, in which a polymer film that is readily carbonized, such as polyimide, is burnt to obtain a carbonized sheet having a thickness of a dozen or more micrometers.
iii) A liquid phase method, in which granular natural graphite or artificial graphite is chemically treated to obtain thin-layered pieces.
Of these, the liquid phase method can provide a great amount of graphene or thin layer graphite pieces relatively in a short time. The liquid phase method can relatively easily control the amount of addition of oxygen-containing functional groups (i.e., the degree of oxidation) during processes including a reduction process, thereby producing partially oxidized thin layer graphite pieces having a desired degree of oxidation, and thus has been receiving attention. The graphene or thin layer graphite obtained by this method is in the form of pieces. Being difficult to use as it is, the graphene or thin layer graphite is handled in the form of a dispersion of pieces and used in various applications. For example, a layered film made of pieces from a dispersion laminated on top of each other is variously used. Uses in the following several applications are expected.
Conventionally, in the fields of semiconductors and electronics, electric wiring elements made mainly of rare metals have been previously developed, and large numbers of excellent products have been marketed. Rare metals, however, are expensive for their limited availability, and it is desired to reduce the amount used for electrodes and circuit materials as much as possible. In addition, heat generation accompanied by more compact wiring arrangements and many other problems have recently been becoming evident. As an alternative to such metals, or as a material that dissipates heat generated, carbon materials are particularly expected to be used. The production of elements and devices from carbon materials by nanotechnology has found to have the potential to provide apparatuses produced from the metal with new functionalities, such as flexibility, lightness, improved mechanical strength, and a reinforced structure, and achieve material replacement associated therewith. Furthermore, carbon materials, for their properties metals do not have, such as sliding properties and biocompatibility, have been increasingly used in a wide variety of applications.
Continuing the description, in the field of household electronic appliances, the recent further miniaturization of computers focusing on improved performance and portability has encouraged the widespread use of information terminal devices, such as tablet computers, cellular phones, and smartphones. Nevertheless, rare metals have gradually been becoming difficult to obtain. For the problem of heat generation, which has been becoming evident, it is desired to develop readily available electrically conductive materials and thin and efficient heat sink materials that can be used in small devices. These materials are used also for solid films and circuits having electrical conductivity at some parts and insulation properties at other parts. The information terminal devices have display units having excellent display functions, and the units are made of materials sensitive to moisture and oxygen. To provide long-term durability, there is a need for materials having gas barrier properties against oxygen and water vapor. For the gas barrier properties, there is a need also in high-pressure gas (e.g., natural gas, hydrogen gas, and other gases) tanks used in natural gas vehicles, fuel cell vehicles, and other vehicles, which are very effective in settling environmental issues; gas supply hoses; flexible pipes in hydrogen gas stations; and other applications.
Regarding portability, numbers of highly accurate portable sensor devices have been developed with which anyone can check the surrounding environment and his or her condition anytime anywhere. Portable sensor devices are broadly classified into physical sensors and chemical sensors that can detect ions, molecules, such as enzymes and DNA, gases, and other substances. The latter chemical sensors are capable of recognizing target substances such as ions and molecules and converting and amplifying signals. In particular, in signal conversion, which is the heart of chemical sensors, an electrode in the form of a flat film connected to a transducer that amplifies signals receives ions and molecules. Signals include light signals, such as fluorescence; signals from measurements of mass changes and thermal outputs; and electrical signals, such as membrane potential and oxidation-reduction current. These signals are received based on the mode of each signal, and measurement results are output. In particular, an electrode material portion that serves to convey electrical signals between the film portion and the transducer is incorporated, for ease of measurement, into a card or chip in which a sample contact portion and the electrode material portion are integrated. The electrode material portion is used once or several times at most and disposed of without being reused. Since the electrode material portion is used once or several times at most, several spare elements for subsequent use are typically provided at hand. In selecting an electrode material, there is preferably no need to consider how long, by whom, and where the element is used. For this reason, the element needs to produce a constant stable performance over a long period of time, allow stable measurement even after being attached to and detached from a measuring apparatus for several times, and be inexpensive and readily available. Thus, there is a need to develop elements having a sample contact portion and an electrode material portion less prone to reduction in performance after long-term storage and use.
A description will now be given of the related art from the viewpoint of electrical conductivity, heat sink properties, and gas barrier properties.
There has been disclosed a technique for forming an electrically conductive layer from a paste made of a carbon material and a binder, which electrically conductive layer is used as an electrode element of an electrochemical sensor (see Patent Documents 1 and 2). In this technique, an electrically conductive material, such as carbon black, and a binder made mainly of a resin are mixed and dispersed to prepare a paste, and the paste is applied to a substrate by screen printing or other methods to form an electrically conductive layer, which is used as an element. This technique can form a good electrically conductive layer but, on the other hand, from the viewpoint of the application of carbon black, requires a large amount of binder in preparing a paste so that the paste is unbreakable. Such a large amount of binder tends to reduce the contact frequency between carbon black particles, resulting in an electrically conductive layer with low electrical conductivity. Furthermore, the electrically conductive layer and the substrate are poorly bonded and readily separated, and thus there is a need to use a special material less smooth and difficult to handle. Despite the use of such a special material, the electrically conductive layer is readily peeled off, has low and variable electrical conductivity, and is poor in handleability and quality stability.
There has been disclosed a technique for applying a dispersion of carbon material pieces to form an electrode for electrochemical measurements (see Patent Document 3). In this technique, a dilute homogeneous aqueous dispersion of graphene oxide prepared mainly by a chemical method is thinly applied to a metal layer used as an electrically conductive layer, not across electrodes made of the metal layer, and a pattern is formed. By the action of the graphene oxide to facilitate the transportation of redox species to the surface, higher sensitivities in electrochemical measurements are achieved. The graphene and the metal layer are weakly adsorbed to each other via the hydrophobic interaction between a thiol compound covalently bonded to the metal layer and the graphene. However, depending on the stronger action in a measurement environment, for example, the change in temperature and the change in the amount of acid/alkaline, the adsorption between the graphene and the metal layer is difficult to maintain, and the graphene is readily peeled off. This results in an element with low long-term stability, and the element can be used in very limited conditions. In addition, the element tends to indicate variable values upon abrasion or even slight peeling and thus is difficult to handle.
Furthermore, there has been disclosed a technique for fine processing of an electronic device using thin graphene flakes (see Patent Document 4). This is a basic technique for patterning a very thin graphene layer utilizing the affinity between a specific portion of a substrate hydrophilized in advance and graphene. The technique, which is intended for use for semiconductor materials and transparent electrodes, can theoretically be applied only to thicknesses of graphene of one layer or two to three layers.
Regarding the heat sink material described above, there has been disclosed a technique for a heat radiating plate produced by rolling graphite into a thin plate (see Patent Document 5). In this technique, expanded graphite obtained by heat expansion is rolled into a thin plate, and then aluminum foil is tightly laminated all over via an adhesive layer to form a heat radiating plate, thereby obtaining a light and bendable material. However, since graphite is rolled, there is a limit as to how thin the heat sink can be, and, in addition, the heat radiating plate has poor conformability to fine shapes because of the metal exterior. Furthermore, the aluminum foil and the thin-plate graphite layer are intervened by the adhesive layer having low affinity, and therefore the adhesion is still poor.
Regarding the gas barrier material described above, there has been disclosed a technique for a composite sheet containing flake graphite (see Patent Document 6). In this technique, the composite sheet is made of flake graphite and layered silicate irregularly superposed on each other, and thus the sheet itself can be used as a self-supported film. However, the interaction between the thin-layered graphite and the layered silicate is weak, and the composite sheet has low resistance to long-time water vapor exposure. Furthermore, when a synthetic resin is used as a support, and the composite sheet is formed thereon, the sheet may readily be peeled off because of the low adhesion to the synthetic resin.